As a result of the recent trend toward performance increase and power increase in automotive engines, the conditions under which valve seats are repeatedly knocked by the valves at high temperatures are becoming severer increasingly. Hence, there is a growing desire for an improvement in the wear resistance of valve seats themselves.
Most of the current valve seats are made of sintered ferrous alloy materials. Examples thereof include a copper-infiltrated sintered ferrous alloy material comprising an Fe matrix containing alloying elements, e.g., Co and Ni; C--Cr--W--Co--Fe or Fe--Mo hard particles dispersed in the matrix; and copper infiltrated in pores in the matrix, so as to maintain wear resistance, as disclosed, e.g., in JP-A-59-25959 and U.S. Pat. No. 4,505,988. (The term "JP-A" as used herein means an "unexamined published Japanese patent application.")
Although the conventional valve seats made of the above-described sintered ferrous alloy material has certain wear resistance, it tends to cause considerable wear of valve seats themselves and valves when used in automotive engines especially of the high-load high-rotation-speed type because of the great knocking and sliding impacts caused by the valves.
This is because the impacts caused by valve knocking is concentrated on hard particles to cause the hard particles to break and fall from the iron-based matrix. Wear thus proceeds and, at the same time, the hard particles which have fallen from the matrix attack not only the valve seats but also the valves to accelerate the wear of both.
Furthermore, since a combustion gas in high-load high-rotation-speed engines has high temperatures, metal adhesion is apt to occur due to the sliding impacts caused by the valves, and thus the wear of both the valve seats and the valves tends to become severer.